In industrial installations using screwed connecting elements, a pull is exerted on the said elements during clamping and releasing operations by pull is exerted on the said elements by means of a tensioning machine for the purpose of keeping the elements in the prestressed state.
This applies to pressurized vessels, such as vessels of nuclear reactors, where the cover is fastened removably to the vessel, in order to allow periodic refuelling of the nuclear reactor and, via this cover, inspection of the interior of the vessel and the elements located inside it.
For this purpose, the cover is fastened to the vessel of the reactor by means of pins which are screwed into the flange of the vessel and which penetrate into smooth holes made in the flange of the cover. These pins are usually used in somewhat large numbers and are relatively close to one another, with the result that the removal and refitting of the cover requires the unscrewing and screwing of all these pins after the prestressed state obtained by means of the tensioning machine has been relieved.
Tensioning machines composed of a set of hydraulic tensioners fastened and distributed in polar symmetry on a carrier ring are known, for example, from FR-A-2,329,414 and FR-A-2,274,997. The hydraulic tensioners act simultaneously and ensure the desired prestress on each pin.
These machines generally have a bulky infrastructure consisting of a one-piece carrier ring, thus making it impossible to install them when the vessel of the nuclear reactor is located in a relatively restricted environment surrounded by non-removable elements.